The MOST project is an NIH-sponsored, multicenter, 5-year study in 2000 patients, designed with three, indivisible and equally important aims: 1) to determine whether an initial strategy for dual-chamber, rate-modulated (DDDR) pacing in all patients with sick sinus syndrome (SSS) improves event-free survival; 2) to determine whether patients randomized to DDDR pacing have a better quality of life than do patients randomized to ventricular, rate-modulated (VVIR) pacing, and 3) to determine if, after 1.5--4.5 years of longitudinal followup, DDDR pacing is more cost- effective than VVIR pacing. The primary endpoint of the study will be a composite clinical endpoint consisting of stroke or all-cause mortality. Total mortality and a composite endpoint consisting of the first occurrence of stroke, heart failure hospitalization, or death will constitute important secondary clinical endpoints. A Quality of Life (QOL) substudy is being conducted, in a randomly selected subset of 1400 patients, to provide data to reverse current practice patterns which appear to restrict access to dual chamber pacing in women and the aged. This QOL study targets women and the elderly to determine if DDDR pacing in superior in these two subgroups. Thus, MOST seeks to determine if the greater initial cost of DDDR pacemakers is offset by preventing adverse events and by improvement in quality of life (patient Benefits), and if DDDR pacing of all patients with SSS includes societal benefits (balancing the increased technological costs of DDDR pacing with potential long-term clinical savings).